OF THE COUNTIES OF NORTHUMBERLAND AND DURHAM. 257 
of upper valve rather angulated, furnished with a large 
triangular button ; surface of convex valve covered unth a 
few distant spines. 
In Magnesian-limestone ; rare. Dalton-le-Dale, Tunstall. 
This Productus is the rarest we have in the limestone. In ge- 
neral form it somewhat resembles the young of the preceding 
species, but may be readily distinguished from it by the large 
flatttened beak of the lower valve. It never attains to the size 
of the former species, and though the muscular impressions are 
similarly arranged, yet they are not so deep. 
The P. latirostratus forms a passage from the P. horridus to 
the following species, which have a very flattened beak, a distinct 
close deltidium, and two condyles on the hinge-line of the 
lower valve. 
3. P. sprnrrerus, King. 
Productus spiniferus, Geol. Russ. i., p. 223 2 
In Magnesian-limestone ; not uncommon. Humbleton, Tun- 
stall, Dalton-le-Dale, Clack’s Heugh. 
This species, which is rather common at Humbleton, has the 
spines of the convex valve very much pressed against and lying 
along its surface. The surface of both valves is covered with 
fine wrinkled, radiating striz. This is the only species of Pro- 
ductus found in the Permian system with a striated surface. 
4, P. Morristanus, King. 
Productus Morrisianus, Geol. Russ. i., p. 223 4 
In Magnesian-limestone ; not uncommon. Tunstall, Humble- 
ton, Dalton-le-Dale, Garmonsway. 
A very irregular shell with the beak of the convex-valve pro- 
duced and pressed downwards. Both valves are covered with 
close-set spines. It generally occurs with the former. 
5. P. ASPERRIMUS, 7. S. 
Shell small, concavo-convex ; lower valve convex ; beak 
with a small flattered areca, and a closed deliidiwm on the 
upper side; hinge-line straight with two condyloid pro- 
cesses, one on each side of the deltidium: upper valve 
slightly concave ; hinge-line with a small pointed process 
or button and a socket on each side of it to receive the con- 
